The present invention relates to a circuit device for a portable or desktop electronic equipment, and more particularly to a modular surface mount device (SMD) employed with an inductor.
Recently, electronic equipments, in particular portable electronic equipments are demanded to be miniaturized and lightened in weight as well as to have high efficiency and multiple function. The portable electronic equipment uses a battery for driving a circuit in the equipment. The electric current for driving the circuit is direct current and/or alternating current. The voltage for driving the circuit is different with the type of the electronic equipment. Therefore, a small circuit device which can be arranged to supply various voltage is demanded.
The portable electronic equipment mostly has an electroluminescent (EL) cell so as to clearly indicate keys at night. The EL cell is driven by alternating current. Therefore, direct current of the battery is converted into alternating current by an inverter and boosted for driving the EL cell.
On the other hand, in the portable electronic equipment, particularly in a portable office automation such as a notebook, direct current of the battery is directly converted by a dc-to-dc converter, or alternating current is once converted into direct current and converted by a dc-to-dc converter to a direct current voltage for driving the equipment.
FIG. 18 shows an integrated circuit (IC) for a conventional dc-to-dc converter employed with a coil. The integrated circuit is an example of a voltage step-up type for obtaining a high output voltage from a low input voltage (battery voltage).
An IC-chip IC comprises a reference voltage circuit Vref, a voltage divider having resistors R1 and R2, a comparator COM, an oscillator OSC, a control circuit CTR (such as PFM=Pulse Frequency Modulation or PWM=Pulse Width Modulation), and a transistor M1.
When the transistor M1 is turned on, energy of a battery V is stored in a coil L1. The stored energy is applied to a capacitor Cl through a diode D and stored therein, thereby obtaining a desired output voltage Vout.
An output voltage of the voltage divider and a reference voltage of the reference voltage circuit Vref are applied to the comparator COM. The input voltages are compared in the comparator COM, and a comparison signal is applied to the controller CTR. The controller CTR produces a control signal which is applied to the transistor M1. The transistor M1 is controlled by the control signal to change an on-off interval (pulse frequency modulation) or an on-off time (pulse width modulation) so that the charging current i1 to the coil L1 and the discharging current i2 from the coil L1 are controlled for obtaining a desired direct current voltage which is higher than the input voltage V. The obtained direct current is applied to a load such as a motor or a printer.
As shown in FIG. 18, the dc-to-dc converter has, other than the IC-chip IC, an external part including the coil L1, diode D and capacitor C1. If the IC-chip IC includes the diode D therein, the external part has only the coil L1 and the capacitor C1.
Also in the case of voltage step-down type for obtaining a low output voltage from a high input voltage (battery voltage), the structure of the integrated circuit is approximately the same as that of the above mentioned circuit.
In order to reduce the thickness of electronic part employed in the portable electronic equipment such as a portable telephone, camera or notebook, it is desirable to compose it by the surface mount technology. On the other hand, an inductor of a thin chip type has small size and high accuracy, compared with an ordinary coil. Japanese Patent Application Laid-Open 5-82349 discloses the thin chip inductor.
FIGS. 19 and 20 show the thin chip inductor. The thin chip inductor comprises three planar swirl coils 22A, 22B and 22C mounted on an insulating substrate 21, interposing coating layers 23A and 23B there-between. The insulating substrate 21 is made of ceramic wafer having a low dielectric constant. Each of the planar swirl coils 22A, 22B and 22C is mainly made of copper thin film having low resistance. Each of the coating layers 23A and 23B is made of heat resistance resin having a low dielectric constant. Outer end portions 24A of three coils are electrically connected to each other. Inner end portions 24B of three coils are also electrically connected to each other.
The thin chip inductor comprising three planer swirl coils has a small size and high efficiency. Since the inductor is sealed with the resin of low dielectric constant, it is superior to resistance of environmental characteristic. Therefore, the inductor is proper to a communication equipment used outdoors or mounted in a car in which vibration or temperature largely changes. However, since the number of windings of the swirl coil can not be increased, inductance of the coil can not be increased.
FIG. 21 shows a conventional voltage step-up circuit employed with a coil for driving an EL cell. The circuit comprises an EL cell EL, a drive circuit DIC for driving the EL cell EL, a coil L and a capacitor C. The circuit is formed by combining the individual elements.
Basically, the EL cell driven by alternating current comprises a suitable phosphor layer placed between sheet-metal electrodes, inserted into insulating layers. The alternating current is applied to the electrodes from the drive circuit DIC. When the intensity of electric field of the phosphor layer exceeds a predetermined value, atoms in the phosphor layer is energized to emit light which has an intrinsic wavelength of the atom. The EL cell emits the light if alternating current voltage or voltage of rectangular wave between 10V and 100V is applied to the electrodes. Furthermore, it is known that luminous efficacy of the EL cell differs in accordance with a difference of working process of material of the cell, and voltage and oscillation frequency applied to the electrodes.
The most significant feature of the EL cell is that the thickness is extremely thin and light. Therefore, the EL cell is used for providing back lighting for LCDs. In the portable equipments such as portable telephone and camera which are mainly used outdoors, it is necessary to provide a voltage step-up circuit only for the EL cell. However, if the aforementioned circuit comprising individual parts is used, the portable equipment is prevented from reducing the thickness, weight and size.